Magnetic and elastic properties ofLa2NiO4+x

We present experimental data on elastic constants, magnetic susceptibility, and magnetization for single-crystal ${\mathrm{La}}_2$ ${\mathrm{NiO}}_4$. Strong acoustic anomalies especially for the ${\mathit{c}}_{66}$ mode are observed for the structural tetragonal-orthorhombic transition at 360 K, whereas for the lower structural transition at 80 K the acoustic effects are rather smeared out. With the help of our magnetic measurements, we can construct a spin model describing the essential features of the magnetic structure in ${\mathrm{La}}_2$ ${\mathrm{NiO}}_4$.